This invention relates to a process and an apparatus for cooling and compressing a wet gas rich in carbon dioxide.
A gas rich in carbon dioxide in the context of this invention contains at least 80% mol. of carbon dioxide and even at least 90 mol. % of carbon dioxide.
A wet gas contains water.
As described previously, an innovative process, for compressing wet flows that are rich in CO2, at reduced cost consists in cooling the gas before the compression in such a way as to condense a sufficient amount of water in order to prevent condensation in the compressor, which also prevents the phenomena of corrosion and which opens the way for using a compressor made of carbon steel or low-alloy steel. The presence of an antifreeze agent (methanol in particular) in the cooling water and the wet gas makes it possible to lower the temperature sufficiently before the compression by preventing the water still present from freezing.
The invention lends itself particularly well to the compression of flows devoid of NOx of which it is known that the interactions with methanol are of a nature as to destroy the latter.
When the fluid to be treated is in addition CO2 that is practically pure but wet, a notable improvement can further be made to the system. This entails mixing the vaporised CO2 in order to cool the water and methanol loop with the fluid cooled as such. When the source of wet CO2 is located far away (at least 50 meters away for example) from the CO2 compressor making it possible to convey the vaporised CO2 at the condensation pressure (between 50 and 100 bars a in general, according to the temperatures of the cooling water available), this solution makes it possible to reduce the number of transfer lines between the different units.
As such, if the apparatus treats several sources of CO2, of which at least one is a source of wet CO2 but quasi-pure (CO2 content>98% on a dry basis for example), for example originating from the scrubbing with amines located several hundred meters or kilometers away, it is sufficient to convey liquid CO2 (via pipes or via lorry by providing for regular fillings) from these remote sources. The vaporised CO2 can be mixed with the compressed CO2 before it is transferred via pipes to the treatment unit downstream. Alternatively, the fraction of CO2 can originate from the liquefier that is part of the apparatus, with a portion of the liquid produced being recycled in the form of liquid in order to cool the methanol via vaporisation.
The liquid CO2, if it is conveyed via pipes, can either be conveyed at the condensation pressure and therefore at a temperature close to ambient temperature (pressure>60 bars), which reduces insulation costs, but which imposes a design pressure for the line that is clearly more substantial; or directly at the vaporisation pressure, taking account of pressure drop. As such, for a use around −20° C., the investment of a line operated at 20 bars (but having to be insulated as it is at −20° C.), is to be compared with a line that must not exceed 25° C. but containing gas at approximately 70 bars.
As a basic solution, it is recommended to not expand the liquid CO2 after it is vaporised: it is then available around 20 bars, at a pressure suitable for the “local” transport of CO2 which would originate from the compression unit of the wet gas.
It can also be considered to turbine the gas rich in carbon dioxide after it is vaporised before mixing it with the cooled gas, possibly at an inter-stage of the wet gas compressor.
It can also be considered to sub-cool the feed gas and/or the water-antifreeze agent mixture (for example methanol) with the turbined gas.
EP-A-2335804 and EP-A-2335806 disclose an apparatus according to the preamble of claim 6.